1. Field of the Invention
The present invention is directed to probe-based instruments and, more particularly, relates to a method and apparatus for reducing or minimizing lateral forces on the probe of the instrument during probe/sample interaction.
2. Description of Related Art
Several probe-based instruments monitor the interaction between a cantilever-based probe and a sample to obtain information concerning one or more characteristics of the sample. For example, scanning probe microscopes (SPMs) typically characterize the surface of a sample down to atomic dimensions by monitoring the interaction between the sample and a tip on a cantilever-based probe probe. By providing relative scanning movement between the tip and the sample, surface characteristic data can be acquired over a particular region of the sample, and a corresponding map of the sample can be generated.
The atomic force microscope (AFM) is a very popular type of SPM. The probe devices of the typical AFM includes a very small cantilever which is fixed to a support at its base and which has a sharp probe tip attached to the opposite, free end. The probe tip is brought very near to or into contact with a surface of a sample to be examined, and the deflection of the cantilever in response to the probe tip's interaction with the sample is measured with an extremely sensitive deflection detector, often an optical lever system such as described in Hansma et al. U.S. Pat. No. RE 34,489, or some other deflection detector such as a strain gauge, capacitance sensor, etc. The probe is scanned over a surface using a high-resolution three axis scanner acting on the sample support and/or the probe. The instrument is thus capable of creating relative motion between the probe and the sample while measuring the topography, elasticity, or some other surface property of the sample as described, e.g., in Hansma et al. U.S. Pat. No. RE 34,489; Elings et al. U.S. Pat. No. 5,266,801; and Elings et al. U.S. Pat. No. 5,412,980.
AFMs may be designed to operate in a variety of modes, including contact mode and oscillating mode. In contact mode operation, the microscope typically scans the tip across the surface of the sample while keeping the force of the tip on the surface of the sample generally constant. This effect is accomplished by moving either the sample or the probe assembly vertically to the surface of the sample in response to sensed deflection of the cantilever as the probe is scanned horizontally across the surface. In this way, the data associated with this vertical motion can be stored and then used to construct an image of the sample surface corresponding to the sample characteristic being measured, e.g., surface topography. Alternatively, some AFMs can at least selectively operate in an oscillation mode of operation such as TappingMode™ (TappingMode is a trademark of Veeco Instruments, Inc.) operation. In TappingMode™ operation the tip is oscillated at or near a resonant frequency of the cantilever of the probe. The amplitude or phase of this oscillation is kept constant during scanning using feedback signals, which are generated in response to tip-sample interaction. As in contact mode, these feedback signals are then collected, stored, and used as data to characterize the sample.
Regardless of their mode of operation, AFMs can obtain resolution down to the atomic level on a wide variety of insulating or conductive surfaces in air, liquid or vacuum by using piezoelectric scanners, optical lever deflection detectors, and very small cantilevers fabricated using photolithographic techniques. Because of their resolution and versatility, AFMs are important measurement devices in many diverse fields ranging from semiconductor manufacturing to biological research.
When a cantilever presses against a sample surface with increasing force, such as is the case when the probe is used to indent the sample or to create force vs. displacement curves, the tip of the probe moves laterally due to the cantilever bending. To apply a force to the sample for the purposes of, e.g., obtaining an indentation measurement, the fixed end of the cantilever is moved vertically through a distance Δz with the tip in contact with the sample. The resultant cantilever bending generates a force k·Δz, where k is the spring constant of the cantilever. This force is not, however, applied entirely normal to the cantilever. A component of the force instead is imposed laterally or along the length and/or width of the cantilever. This component was historically deemed to be non-problematic because the lateral component of the applied force vector is typically much smaller than the normal component. However, it has been discovered that the lateral force can in fact be an order of magnitude higher than the normal force.
The reasons for this somewhat counterintuitive characteristic of AFM operation can be appreciated from FIG. 1A, which schematically shows an AFM probe P interacting with a sample S during an indenting operation. The probe P includes a cantilever C having a tip T. The cantilever C is fixed on or formed integrally with a base B. The probe P is typically inclined at an angle α of about 10° to 15° relative to the surface of the sample S in order to assure adequate clearance between the probe holder and the sample and in order to facilitate data acquisition by a probe detector assembly. As the probe-sample spacing in the z direction is decreased (by movement of the probe P toward the sample and/or by movement of the sample S toward the probe P in the z direction) to increase the indentation force, the lateral distance available to the cantilever C in the plane L decreases. This decrease creates a compressive strain along the length of the cantilever C and results in cantilever bending as seen in FIG. 1B. Since the cantilever C has a much higher stiffness along its length in the plane L than perpendicular to its length in the plane N, the majority of the applied force is actually directed in the lateral direction in the plane L. The resulting forces tend to cause the probe tip T to displace laterally so that the tip T engages the sample S at a location EACT that is offset from the point EDES of desired engagement by an offset X as seen in FIG. 1B.
This lateral motion of the tip is undesirable since the resulting lateral forces negatively affect the shape of the surface indentations. In the absence of relative probe/sample measurements, material piles up uniformly in front of and behind the tip as seen in FIG. 2A. For instance, if the lateral forces tend to push the tip T away from the base, sample material tends to pile up disproportionately ahead of the probe tip T as seen in FIG. 2B. Conversely, if the lateral forces tend to pull the tip toward the base, material tends to pile up disproportionately behind the tip T as seen in FIG. 2C. In either event, the indentation is non-uniform, leading to inaccuracies in the resulting indentation data such as errors in the acquired force vs. displacement curves.
Some current AFM indentation tools attempt to reduce the lateral forces on a probe by moving the probe laterally away from the indentation point as the probe-sample spacing decreases. For instance, the Nanoscope software, employed in some microscopes manufactured by the assignee of the present application, includes a correction called an “x rotation” feature which moves the sample or tip laterally in proportion to the vertical or z motion of the sample or tip, whichever is being driven. An AFM 10 configured to perform the function is illustrated in FIG. 3. It includes a probe device 12 configured to indent or otherwise interact with sample S mounted on a support 26. The probe device 12 includes a probe 14 supported on a substrate 16. The probe 14 includes a cantilever 18 bearing a tip 20 that interacts with the sample S. The cantilever 18 includes a base or fixed end 22 extending from substrate 16, and a free, distal end 24 that receives the tip 20. The sample support 26 is movable in the xy plane under operation of an xy actuator 28, for example. Preferably, sample support 26 is also movable in a z direction that is perpendicular to an xy plane of the sample support 26 under the power of a z actuator 30. The z direction is typically vertical. Cantilever deflection is monitored by an optical detection system in the form of a detector 32 that receives light emitted from a laser 34 and reflected from the cantilever 18.
In operation, the cantilever deflection data obtained from the detector 32 is manipulated in a controller (not shown) and used to generate a feedback signal that it is amplified in an amplifier 36 and then used to actuate the z actuator 30, with the resultant drive signal providing information indicative of sample characteristics in a known manner. The controller also transmits a drive signal to the xy actuator 28 via an amplifier 38 so as to move the sample S relative to the probe 14 to affect a scanning operation. The scanning operation typically comprises a so-called raster scan in which data is taken in a first or x direction in a series of lines that are spaced from one another in a second or y direction typically substantially perpendicular to the x direction.
Pursuant to the x rotation technique discussed above, the signal to the xy actuator 28 is modified with an x rotation scaling signal or compensation designed to at least partially compensate for lateral loads on the probe 14 resulting from sample/probe interaction. That modification is shown schematically as taking place in an adder 40 located upstream of amplifier 38. The compensation signal is proportional of the drive signal applied to the z actuator 30. The magnitude of the compensation signal is based solely or at least primarily on the geometric configuration of the AFM and is largely independent of cantilever deflection.
Hence, this approach merely adds an x-offset voltage to the signal from the xy actuator that is proportional to the z-voltage applied to the z actuator 30 without taking into account the properties of the sample, the cantilever, or the interaction between the two. This approach is less than optimally effective for several reasons, particularly if the cantilever 18 is not substantially stiffer than the sample. For instance, a cantilever of a given stiffness will bend more for a given amount of z-travel when the probe is driven against a relatively hard surface than when it is driven against a relatively soft surface, resulting in greater lateral deflection and the need for more compensation. The x-rotation software does not take this variable into account. It also ignores the effects of cantilever motion such as bending on lateral deflection. It also fails to take tip sharpness into account. As a result, “x rotation” control is sometimes ineffective, and, even when it is effective for a particular probe-sample combination, it tends to have low repeatability.
Axially symmetric indenters have been developed that lack the need for lateral compensation. However, these instruments have low mechanical bandwidth (on the order of 300 Hz) and relatively poor sensitivity because they are subject to high levels of noise. For instance, MTS and Hysitron produce nano-indentation devices in which an indenter tip such as a Berkovich tip is driven into a sample using a multi-plate capacitor transducer system. The device has drive and pickup plates mounted on a suspension system. It provides relative movement between the plates when the forces applied to the pickup plates drive the probe into contact with the sample. The change in space between the plates provides an accurate indication of the probes vertical movement. The input actuation forces and vertical position readout are therefore all-decoupled, resulting in a generally purely symmetrical indentation process. In practice, the sensor element is mounted on a scanning tunneling microscope, and a sample is mounted on the sensor. The force sensor then can be used for both measuring the applied force during micro indentation or micro hardness testing and for imaging before and after the testing to achieve an applied AFM-type image of the surface before and after the indentation process. Systems of this type are described, e.g., in U.S. Pat. No. 5,576,483 to Bonin and U.S. Pat. No. 6,026,677 to Bonin, both assigned to Hysitron Incorporated.
While the indenter described above provides axially symmetric indentation, it has a very low bandwidth because of the relatively large mass of the capacitive plates. The instrument also cannot obtain an accurate image of indentations, particularly in relatively elastic samples, because of sample rebound between the indentation and image acquisition passes and because of the large tip radius inherent in the indenter tip. It also has relatively poor force sensitivity, on the order of 15 nano-Newtons, as opposed to a few nano-Newtons for a true AFM having a much smaller tip.
The need has arisen to effectively and reliably reduce the lateral forces imposed on a probe as a result of probe-sample interaction.